Does the Zendrive just want to see a buffer?

[midwayfair]

Something I noticed: The Zendrive's volume level does not scale linearly with the input signal. Turning down from 10 to 9 on the guitar goes from a boosted signal to one that's below unity.

If I turn on my compressor in front of it, though, and all of a sudden the entire pedal is louder in general, and the volume seems to scale identically to the bypassed signal.

What gives? I have some experience with pedals that don't clean up linearly, but very little with single-IC-based overdrives in general (playing or building). Is this some sort of gating, or does it simply have something to do with the input signal hitting the diodes hard enough? Something else?

It sounds nice either way, and I use a buffer all the time so it doesn't affect its functionality for me with normal use, but I'm just trying to get some more understanding.

[Jabulani Jonny]

I know I'm resurrecting, but as someone who's getting ready to build a Zendrive for a buddy, I'd love to see some input on this.  Lazarus...come forth!

[culturejam]

Might depend on the type of pickups and the value of the guitar volume pot.

After doing some reading recently on input impedance and pickup loading, I'm beginning to think that the typical 1M pulldown resistor really isn't enough (value-wise) for all scenarios. I've started to use 2M2 on my projects.

[midwayfair]

Probably worth revisiting this post now that I know a bit more about this stuff.

I went back to my original Zen Drive clone and added an input buffer (with buffered bypass!). I'm going to say that I'd never build the thing again without a buffer. I don't think it reacted much like an amp at all as far as the input loading goes, though I get that the distortion, and not the "clean" settings, were meant to be an amp imitation.

The op amps used in the Zen are BJT-based. BJTs just in general don't have good input impedance. This is compounded when you're using the input stage to also serve as a gain and distortion stage. I know that the Son of the Screamer and similar designs are battle tested and that some people like them, but to me this method just makes the guitar sound muffled and behave erratically.

@ Forrest: One good way to think of total input impedance calculations is to take into account average cable lengths. I'll refer to PRR's post in this thread:
http://www.diystompboxes.com/smfforum/index.php?topic=105399.0

Everyone should at least look at that thread for a second, because PRR's post contains one of the most awesome graphics I've ever seen.

Anyway, this is what he said:

Two 2Meg inputs (1Meg mono) ought to be plenty gentle for any signal on a wire. (Three feet of cable is 100pFd which is under 200K at the top of the guitar band.

This was really useful for actually visualizing a real-world consequence of input impedance. A decent rule of thumb to derive from this would be 200K for every three feet of cable you expect someone to be using before you start cutting into frequencies produced by the guitar (realistically, that's about ~15KHz; most adults can't hear above 17Khz at all).

Keep in mind, though, that BJTs and FETs have different input impedance characteristics, especially when it comes to source/emitter vs. collector/drain (voltage gain) followers. Also, MOSFETs make essentially perfect single-stage drain followers with super low input and output impedances (better than even FET input op amps, imo), but make crappy source followers.

[culturejam]

Or you can just use a buffer and not worry about all the math. 

[midwayfair]

Or you can just use a buffer and not worry about all the math. 

The 200K/3 feet math still applies to buffers, it's just that they really simplify the number of moving parts!

[Jabulani Jonny]

Jon: did you use the frequencycentral  layout for the Zendrive or the MB Serendipity?

I assume it's fairly easy to make it buffered bypass.


Sent from my iPhone using Tapatalk

[midwayfair]

Jon: did you use the frequencycentral  layout for the Zendrive or the MB Serendipity?

I built it on perfboard (my own layout).